This invention relates to a process and a device for sensing and evaluating knocking combustion during operation of an internal combustion engine. In a combustion chamber of the engine, the electromagnetic radiation, which changes with knocking combustion, is detected and transmitted to a measuring and/or processing device.
The actual efficiency of internal combustion engines is largely dependent on the compression ratio. In general, an increase in compression ratio results in significantly reduced fuel consumption, particularly in the part-throttle region, based on values of current production internal combustion engines. However, this increase is limited by the occurrence of knocking combustion with a high engine load.
In engine technology an abnormal combustion phenomenon is referred to as "knocking" which is caused by an extraordinarily fast reaction of the combustible mixture compared to normal combustion of engines and arises in the latter interval of the combustion phase. The precise processes that take place during knocking combustion are, to this day, not clearly explained. The most widespread theory is that knocking involves auto-ignition processes in the mixture which has not yet been touched by the flame. Following auto-ignition, the velocity of the flame can be in supersonic range relative to sonic speed in the burned mixture and in the unburned mixture. Furthermore, the damage typically caused by knocking is indicated by shockwaves which arise during knocking combustion.
In the definition of the term "knocking", no distinction is usually made between the first auto-ignition phenomenon with subsequent fast reaction of the combustible mixture, and the thereby induced oscillations of the burned and unburned mixture in the combustion chamber. Thus, the fast reaction of the combustible mixture constitutes the primary effect, and the subsequent oscillations of the burned and unburned mixture in the combustion chamber constitute the secondary effect.
If an internal combustion engine is operated with knocking combustion, it can result in the destruction of the engine. In order to assure destruction-free operation, makers of internal combustion engines must, when setting the ignition point, either maintain a greater distance from the knock limit or use a knock control system, particularly due to varying qualities of fuel and due to variances in the compression ratio in the assembly production, the ignition process, etc.
If such control system is used, it is not necessary to maintain the aforementioned safety distance. However, it is desirable to use a suitable device and a suitable process during engine operation to recognize or decide whether or not there is any knocking. Then, the corresponding operating parameters are adjusted in such a manner via a corresponding control unit that the engine is operated directly on the knock limit. When this condition is fulfilled, the goal is to achieve maximum efficiency in an engine with high compression ratio.
Several processes in and devices for the sensing of knocking are known with which one or more physical values or processes that change only during knocking or arise only during knocking can be detected.
German Patent Disclosure Document DE-OS 31 08 460 discloses a process in which the electromagnetic radiation, emitted during combustion, is evaluated relative to the oscillations that occur upon knocking. However, here the secondary effects of knocking combustion, namely oscillations, are detected. Since these secondary effects are dependent on the structure of the engine, i.e., the shape of the combustion chamber, etc., and the location of the sensor in a controlled system, the data relating to the overall valid knock intensity, which is transferrable to various engines, are not problem free.
German Patent Disclosure Document DE-OS 29 32 193 relates to a device in which, to obtain a higher thermal efficiency, ionization probes are used as flame front probes of which at least two per cylinder must always be available to detect the travel time of the flame and thus the flame propagation velocity. The drawback with this approach is that the error potential with the use of two probes is very high since the propagation mechanism of the flame is not precisely known and is not constant. The use of many ion current probes does not make it easier to determine the speed of the flame. Moreover, this prior art approach presents a drawback because of its high cost of production. Another drawback of the ion current probe is that due to the shock waves, arising during knocking combustion, the fine wires of the ion current probes are rapidly destroyed and thus the device is more susceptible to failure.
German Patent Disclosure Document DE-OS 31 10 996 relates to a sensor system for detecting physical parameters in the combustion chamber of an internal combustion engine in which the end of a receiver, which is subjected to the comustion chamber, is thickened in the shape of a mushroom. This approach does not restrict but rather increases the sensed volume. Such a system is suitable primarily for detecting secondary effects, in particular the oscillations of the fuel, induced by auto-ignition, in the combustion chamber. However, if the sensor exhibits a cavity similar to a heat pipe, essentially only the processes in the cavity, or via several openings over a wide range of the main combustion chamber, are observed.
In U.S. Pat. No. 4,393,687 the sensing of the combustion chamber is via a pre-chamber in which the mixture which is contained there is spark-ignited. Thus, there are totally different conditions compared to direct sensing of the main combustion chamber of an internal combustion engine, since the igniting processes in the pre-chamber superimpose the signals emitted from the knock phenomenon in such a manner as to interfere with a precise measurement. Moreover, the region of the cylinder loading, which is detected at the earliest after spark ignition of the flames, is observed by a sensor. However, this region is already completely burned when the knocking begins so that no primary effects of knocking combustion occur here.